New `distance_feature` query type in ES7

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New query type distance_feature
https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-distance-feature-query.html

We currently use geo_distance for filtering and _geo_distance for scoring, could this replace one or both of those?

Needs some investigating

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We also currently use function_score for focus queries.

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I suspect that if this is using a KNN algorithm that it could be significantly faster than using function_score for computing focus.point queries.